1. Field of the Invention
This invention relates to a recording medium, more particularly to a perpendicular magnetic recording medium.
2. Description of the Related Art
As technology advances, higher recording density of a recording medium is required. To increase the recording density, minimization of the volume of each of recording units of the recording medium has been proposed. However, when the recording unit gets smaller, the product (KuV) of the perpendicular anisotropy constant (Ku) and the switching volume (V) of the recording unit will become insufficient to overcome thermal disturbance caused by exterior temperature, thereby resulting in superparamagnetism attributed to unstable magnetic moment.
To solve the superparamagnetism problem, a perpendicular magnetic recording medium has been proposed. In the perpendicular magnetic recording medium, since the magnetic moment is perpendicular to a magnetic recording layer, thermal stability can be maintained even though the volume of the recording unit is reduced.
FIG. 1 shows a conventional perpendicular magnetic recording medium 1 including a substrate 11, a soft magnetic film 14 formed on the substrate 11, a non-magnetic film 13 formed on the soft magnetic film 14, and a magnetic recording film 12 formed on the non-magnetic film 13. The non-magnetic film 13 includes an interlayer 131, a buffer layer 132, and a seed layer 133 formed on the soft magnetic film 14 from bottom to top in this sequence.
In general, in order to minimize the volume of a recording unit, the magnetic recording film 12 made from a granular Co-based material, such as CoPtCr—SiO2, issued. In CoPtCr—SiO2, SiO2 is segregated at CoPt grain boundaries, Pt enhances perpendicular anisotropy constant (Ku) of the magnetic recording film 12, and Cr is used to reduce undesired recording medium noise. Therefore, the size of the CoPt grain can be minimized and signal interference among the grains can be reduced, which results in an increase in signal to noise ratio (SNR) of the magnetic recording film 12. In addition, since Co element has an easy axis parallel to a c-axis of the lattice, in order to permit CoPt grains in the magnetic recording film 12 to grow in a fixed orientation texture, i.e., (0002) texture, along a direction (X) shown in FIG. 1 so as to improve the perpendicular anisotropy constant, the seed layer 133 is usually made from a material having a hexagonal close packed (hcp) crystal structure.
In addition, since (111) plane of face centered cubic (fcc) crystal structure exhibits the best lattice match with (0002) plane, the buffer layer 132 made from a non-magnetic material with a fcc structure is provided beneath the seed layer 133 so as to control the crystal growth orientation of the seed layer 133 and the magnetic recording film 12 along (0002) plane.
Theoretically, grain orientation of the magnetic recording film 12, the seed layer 133, and the buffer layer 132 is mainly controlled by interfacial strain and tensile stress. Specifically, reduced interfacial strain achieved by providing good lattice match at the interface of two films/layers and higher tensile stress favor growth of (0002) orientation texture. To improve desired orientation texture of these film/layers and thus the perpendicular anisotropy constant, the interlayer 131 is provided. In present use, amorphous nickel-phosphorus alloy or cobalt-zirconium alloy is used as a material for the interlayer 131. However, improvement of the desired orientation texture to the magnetic recording film 12, the seed layer 133, and the buffer layer 132 provided by such interlayer 131 is limited. Hence, there is a need in the art to provide a perpendicular magnetic recording medium having an interlayer that can improve the growth of (0002) orientation texture and thus enhance the perpendicular anisotropy constant as compared to the prior art.